A power supply system that is equipped with a plurality of power supply devices converting inputted electric power and outputting the converted electric power and is configured to supply the outputted electric power to an external load is known. As one of the power supply systems of this type, a power supply system described in Patent Document 1 is capable of, even when the state of one of the power supply devices becomes abnormal (the abnormal state), supplying electric power required by the external load by using the other power supply device.    [Patent Document 1] Japanese Unexamined Patent Application Publication No. JP-A 7-213060
Before a power supply device gets ready to output a sufficient level of electric power after input of electric power into the power supply device starts, a given delay time is required. Therefore, in order to avoid shortage of electric power supplied to an external load when the state of the power supply device becomes abnormal, the power supply system described above previously inputs electric power to all of the power supply devices. In other words, the power supply system described above operates in hot-standby.
On the other hand, the efficiency of conversion of electric power by the power supply device (the electric power conversion efficiency) decreases when load on the power supply device (i.e., the level of electric power outputted by the power supply device) becomes excessively small. Moreover, as mentioned above, the power supply system described above supplies electric power outputted by all of the power supply devices to the external load. Therefore, before the state of any of the power supply devices becomes abnormal, load on each of the power supply devices is relatively small. Consequently, in the power supply system described above, there is a fear that the electric power conversion efficiency excessively decreases.